1. Field of the Invention
The present invention relates to metal-air or metal-oxygen electrochemical cells and to a method of operating such cells as a storage battery and, more particularly, to metal-air or metal-oxygen electrochemical cells and to a method of charging such cells so as to improve the cell performance.
2. Description of the Prior Art
Metal-air or metal-oxygen electrochemical cells require for their operation the consumption of oxygen at the positive electrode during cell discharge. In what follows the term metal-oxygen will be used generally to also include metal-air.
A typical metal-oxygen cell includes a vessel which contains an aqueous solution of an alkali or alkaline earth base, preferably KOH. Immersed in the electrolyte is a pair of electrodes. The negative electrode may be made of various metals, including, but not limited to zinc, cadmium and iron while the positive electrode may be made of any convenient material. In normal operation, when the cell is being discharged, anions, typically hydroxyl ions, travel to the negative electrode and react there with the metal, typically Fe, Zn or Cd, to yield electrons. At the positive electrode, injected oxygen reacts with the water of the electrolyte to form the anions.
For example, where an iron electrode is used as the negative electrode and where a base, such as KOH, is used as the electrolyte, electrochemical processes during cell discharge at the two electrodes can be described by the following reactions. The reaction at the negative electrode during discharge is: EQU Fe+2OH.fwdarw.Fe(OH).sub.2 +2e.sup.- ( 1)
The reaction at the positive electrode during discharge is: EQU 2e.sup.- +1/2O.sub.2 +H.sub.2 O.fwdarw.2OH.sup.- ( 2)
The summary reaction, combining reactions (1) and (2) is: EQU Fe+1/2O.sub.2 +H.sub.2 O.fwdarw.Fe(OH).sub.2 ( 3)
It is seen that in order to make reaction (2) take place it is necessary to provide oxygen to the positive electrode. Oxygen is usually provided either as pure oxygen or in an impure form, most commonly as air.
Once such a cell is sufficiently discharged, it becomes necessary to recharge the cell before it can do any more work. Charging is accomplished by superimposing on the electrodes an external voltage of a sufficiently large oppositely directed electric potential so as to force the above reactions, (1) and (2), and therefore also (3), to take place in the reverse direction. It can be seen that oxygen is evolved in the course of carrying out reaction (2) in the opposite direction during charging.
The method of operation of the cell described above suffers from a number of basic disadvantages. First, the evolution of oxygen during the charging of the cell leads to the deterioration of the positive, or oxygen, electrode, including the loss of catalytic properties of the electrode, which loss permanently decreases the maximum current which the cell may produce during subsequent discharge. Oxygen evolution also indirectly harms the metal electrode because of the heat dissipation and consequent temperature increase in the cell which occurs in the charging process as a result of the parasitic reactions wherein the metal electrode reacts with oxygen and water to ionize to form the metal hydroxide, which reaction is highly exothermic.
Second, when air rather than pure oxygen is used in the discharge process, CO.sub.2 contained in the injected air is absorbed in the electrolyte solution and is subsequently consumed, causing the KOH concentration in the electrolyte to decrease. The reaction is: EQU CO.sub.2 +2KOH.fwdarw.K.sub.2 CO.sub.3 +H.sub.2 O (4)
While this is unavoidable under either the presently known processes or under the present invention, the importance of this reaction during charging is eliminated according to the process of the present invention. Conventional charging leads to the release of oxygen which bubbles to the surface and agitates the surface of the electrolyte solution thereby causing the absorption of air, containing some carbon dioxide, into solution. According to the present charging scheme, there is no evolution of gases and thus less opportunity for air, and carbon dioxide, to get entrained into the solution.
For these and other reasons which will be elaborated upon below, it is thus undesirable to charge a metal-oxygen cell in the conventional method. Several partial solutions have been proposed in an attempt to overcome some of these problems.
One approach has been to provide a third electrode which will be used exclusively for charging the cell. In this manner the main electrode is used only for discharge. Once the cell has been sufficiently depleted, the main electrode is disconnected and a third electrode is hooked up to the voltage source. Charging is accomplished through this third electrode which can be constructed so that it does not deteriorate from exposure to evolved oxygen during charging operations. Since the main negative electrode is not involved during charging, its deterioration is significantly slowed and it enjoys a longer life.
Another partial solution involves the fabrication of a complex and costly positive electrode which has pore structures designed such that oxygen can easily be pushed out to the electrolyte during cell discharge but that oxygen produced during charging is unable to reenter the electrode or penetrate to any great extent into the electrode and is therefore forced to travel along the electrode surface, thus adversely affecting only a small part of the electrode.
Both of the above solutions suffer from significant disadvantages. Both introduce additional costs in terms of capital and operating costs.
There is thus a widely recognized need for a simple and cost-effective method of charging metal-oxygen electrochemical cells which will not require the use of special or additional electrodes and which will still significantly increase the life of the cell electrode. It would be desirable to have a method of charging a metal-oxygen cell which would not expose the electrodes to oxygen.